The invention relates to the preheating of stone or sized particulate material such as limestone which is being fed to a rotary kiln calcining system. The invention is also applicable to similar calcining or gas/solid heat transfer processes.
A rotary kiln calcining device has relatively low thermal efficiency, particularly with highly endothermic processes such as the burning of lime. Rising fuel costs have increased the economic feasibility of preheating devices for commercial rotary kilns.
These preheating devices are based on convection heat transfer with the hot gases exhausting from the kiln being passed through voids in the feed material as in a vertical shaft kiln. Because of the relatively small size of rotary kiln feeds and the large volume of combustion gases, impractically high pressure drops will be developed in the system unless the beds are relatively wide and shallow. The gases should have uniform motion across the entire bed to avoid overheating or fusing.
Because of the relatively higher heat transfer efficiency of preheaters, the calcining reaction tends to move from the rotary kiln into the preheater resulting in higher kiln exhaust gas temperatures. These high temperature gases result in high surface temperatures on the stone feed fed to the kiln, with the result that the preheater has little cooling effect on the gases. The cumulative effect is that a circulating heat stream is set up at the juncture of the preheater and kiln outlet, the high temperatures increasing heat losses and mechanical difficulties.
In the conventional preheater the kiln exhaust gases are introduced directly to the preheater discharge area. The resulting high temperatures create great radiation and efficiency losses. A cumbersome feeding mechanism is required to be effective over the major portion of the bed area, this mechanism operating at unusually high temperatures. Furthermore, the short time interval during which the feed material is subjected to high temperatures results in only the outer surface portions of the stone being heated, the centers remaining at comparatively low temperatures.
It is a general object of the present invention to overcome the above difficulties of conventional preheating devices. Briefly, the invention comprises a moving bed of material so arranged that the kiln exhaust gases are introduced at a level in the preheater considerably above the feeder which discharges the stone to the rotary kiln. This creates a "soaking" or holding zone between the gas entry and discharge levels which may have a sufficient retention time to allow the high surface temperatures developed on each particle to be diffused substantially into its entire volume. The result is a preheated material of more uniform temperature with a lower surface temperature but a greater stored heat content. When this material is fed to the rotary kiln it will not have the tendency to return heat to the gas stream but will absorb some heat, maintaining gas temperatures at a lower level.
It is a further object of the invention to provide a novel and improved preheater in which the soaking zone portion couples the entire cross-sectional area of the preheater stone beds to a relatively small and simple feeding device working at moderate temperatures, thus reducing maintenance problems. The conically formed holding zone contributes to the uniform motion of the material and facilitates coupling the large preheater area to the single feeder.
It is a further object to provide an improved preheater of this nature which develops sufficient stone bed surface to allow admission of the gases into the relatively small void areas of the material. The hot gases are introduced around the outer periphery of a circular cross-section and are removed from the center. Since flow resistance is higher for hot than for cooler gases, this arrangement allows the development of stone bed void areas roughly proportional to the gas temperatures to give efficient use of the bed area.